This invention relates to a control circuit for starting and operating gas discharge lamps and, more particularly, to a control circuit of this type which provides automatic current regulation as a function of the lamp current by means of automatic frequency control.
Starting and ballasting circuits are required for the stable and efficient operation of gas discharge lamps. Recent developments in the art of control circuits for discharge lamps indicate that improved operating characteristics are obtainable by operation of the lamps at high frequencies, e.g. at frequencies above about 5 Khz.
Various types of ballast circuits are well known in the art for controlling the operation of gas discharge lamps. For example, U.S. Pat. No. 4,060,751 by T. E. Anderson describes a control circuit for operating a gas discharge lamp utilizing a frequency controlled inverter and a resonant matching network. The resonant circuit consists of an inductor connected in series with the parallel combination of a capacitor and the gas discharge lamp. The discharge lamp is connected as a damping element across the capacitor of an otherwise high Q series resonant circuit. Prior to ignition, the lamp presents a very high impedance so that the Q of the resonant circuit remains high and the circuit is automatically driven at its resonant frequency. A voltage buildup occurs in the high Q circuit to provide the high voltage necessary to initiate a discharge in the lamp. After ignition, the lamp's impedance decreases greatly, thereby loading the resonant circuit and lowering its Q. The inverter then functions as a current regulator in which the inductor of the control circuit limits the current flow through the negative lamp impedance thereby to limit the lamp input power and provide stable operation. An increase in the DC supply voltage produces an increase in the inverter operating frequency and therefore an increase in the impedance of the inductor.
U.S. Pat. No. 4,060,752 by L. H. Walker also discloses a variable frequency ballast circuit providing a regulated, constant output power to a gas discharge lamp. The discharge lamp is again connected in parallel with the capacitor of a series resonant LC circuit. The operating frequency of an inverter or variable frequency square wave oscillator is controlled by a frequency control circuit which is in turn controlled either as a function of the time derivative of the lamp current via a dI/dT sensor or as a function of the amplitude of the lamp current. The control circuit maintains constant power to the lamp via the resonant matching circuit and exhibits an operating frequency which increases as the load impedance increases.
A variable frequency inverter-ballast control circuit for regulating the current in a gas discharge lamp is disclosed in U.S. Pat. No. 3,611,021 in the name of K. A. Wallace. This control circuit energizes the discharge lamp via a leakage reactance transformer in combination with a first capacitor connected across the transformer secondary and a second capacitor connected in series with the lamp and selected to be near resonance with the transformer leakage reactance at the fundamental frequency of a variable frequency square wave inverter. The first capacitor resonates with the transformer leakage reactance at a selected harmonic of the inverter fundamental frequency. The harmonic resonant voltage is added to the transformer fundamental voltage to produce a voltage sufficient to ignite the discharge lamp. After ignition, the equivalent series impedance of the second capacitor and the transformer winding at the fundamental inverter frequency provides the necessary ballast for stable lamp operation. A current sensing circuit senses the level of the lamp current and feeds back an error signal to adjust the inverter fundamental frequency in a sense to maintain the lamp current constant.
U.S. Pat. No. 2,928,994 by M. Widakowich shows a variable frequency inverter whose frequency varies as a function of a DC supply voltage so as to maintain the current in a fluorescent lamp constant despite any variations in the level of said supply voltage.